1. Field of the Invention
This invention relates to a digital modulating method used for recording a PCM audio signal, digital data used in a computer, etc. on an optical disc or other recording medium.
2. Description of the Prior Art
In recording digital data on a magnetic tape, optical disc or other recording medium, the digital data to be recorded is modulated. This modulation is called digital modulation or channel coding. There have been proposed various methods of digital modulation. Parameters for evaluation of digital modulating methods include window margin Tw, minimum length between transition Tmin, maximum length between transition Tmax, recording density DR (density ratio), and so on. Digital modulating methods that have been proposed heretofore are directed to removal of a DC component in a frequency spectrum of a modulated signal (so-called DC free) and an increase of the minimum length between transition Tmin or an increase of the window margin Tw.
For example, EFM (eight to fourteen modulation) employed in a CD (compact disc) converts 8 data bits into 14 channel bits, as described in Japanese Patent Laid Open Publication No. Sho 57-48848. Selected are bit patterns of 14 bits which include two or more "0"s (logic 0) sandwiched between "1" (logic 1) and "1". Bit patterns which satisfy this condition comprise 277 patterns from among (2.sup.14 =16,384 patterns, and these 277 patterns include 267 patterns whose Tmax's do not exceed a predetermined value. 256 patterns from among these 267 patterns are associated with 256 patterns of 8-bit data bits by one to one.
When indicating the data bit interval by Tb, the above-mentioned EFM has the parameters indicated below:
Tw=(8/17)Tb
Tmin=3Tw=(24/17)Tb
Tmax=11Tw=(88/17)Tb
DR=(24/17)
Respective symbols of 14 bits satisfy the condition that two or more "0"s are placed between "1" and "1". In order to satisfy this condition also between symbols, 3-bit coupling words are used. Four such coupling words are (000), (100), (010) and (001). One of four coupling words is determined so as to satisfy the parameters Tmin, Tmax, etc. and to minimize DSV (digital sum value; where the summation is done regarding high levels as +1 and low levels as -1) of data after modulation. By deciding the coupling word according to this rule, low frequency components of the modulated data can be reduced. Therefore, data bits in the form of 8 bit words are finally converted into channel bits in the form of 17 bit words.
The EFM, for example, which is one of the conventional digital modulating methods, provides easy extraction of clocks from modulated data and, for reducing low frequency components of demodulated data, the EFM minimizes the maximum length between transition Tmax. However, in case of recording and reproduction not requiring removal of DC components or in case of no need for clock extraction from modulated data, restriction on Tmax may be alleviated.
Examples of such cases having no, or reduced, need for removal of DC components are a case not passing through a transformer or other element incapable of DC transmission, and a case using an NRZI or other recording and reproducing method in which edges invertible from "0" to "1" or vice versa have information.
On the other hand, there is proposed a method for forming clock pits and tracking servo pits in preformat areas provided for respective segments in one track of a rewritable optical disc such as magnetooptical disc. Since this method supplies a reproduced output of a clock pit to a PLL and extracts a bit clock, the clock extraction is not affected even if the Tmax of data to be recorded in the data area is long. Further, a tracking servo is done stably even in the case having DC components because there are servo pits for tracking.
Additionally, the EFM, which converts 8 bits into 17 bits, involves a problem that the generation of modulation clocks is diffucult. More specifically, a frequency of a clock regarding data bits and a frequency of a clock regarding channel bits are not held in an integer ratio relationship: therefore, the scale of hardware for producing both clocks is large.